Method for diffusing an impurity from a doped oxide of pyrolytic origin



6 J. c. BRIXEY. JR. ETAL 3,354,008

METHOD FOR DIFFUSING AN IMPURITY FROM A DOPED OXIDE OF PYROLYTIC ORIGINOriginal Filed April 15, 1964 ARGON g 2a v Fig. 2b

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- BY WV j iwwk ATTORNEY United States Patent O 3,354,008 METHOD FORDIFFUSING AN IMPURITY FROM A DOPED OXIDE OF PYROLYTIC ORIGIN John C.Brixey, Jan, and Kenneth E. Statham, Richardson, Tex., assignors toTexas instruments Incorporated, Dallas, Tera, a corporation of DelawareContinuation of application Ser. No. 359,883, Apr. 15, 1964. Thisapplication Sept. 21, 1%6, Ser. No. 581,118 9 Claims. (Cl. 148-187)ABSTRACT OF THE DISCLOSURE Disclosed is a method of doping asemiconductor device by depositing an impurity-containing oxide upon thesurface of a semiconductor wafer, and then diffusing the impurity fromthe oxide into the wafer.

This application is a continuation of application Ser. No. 359,883 filedApr. 15, 1964, now abandoned.

This invention relates to diffusing of impurities into semiconductormaterial, and more particularly to a method of diffusing an impurity orimpurities into a semiconductor from a doped oxide of pyrolytic origin.

Prior to this invention semiconductors have been treated with impuritiesunder such conditions that the impurity is diffused into thesemiconductor, modifying its electrical roperties and enhancing itsutility to provide such devices as transistors, diodes, resistors andthe like. When treated with such an impurity, the impurity forms anatmosphere about the semiconductor body, the atoms of the impurityentering the entire surface of the semiconductor exposed to the impuritycontaining atmosphere. To obtain selective area diffusion it isnecessary to mask areas of the semiconductor device. Masking, however,at diffusion temperatures poses many and varied prob lems.

To effect a diffusion mask, it has been suggested that the surface ofthe semiconductor bodies be oxidized. In the case of germanium, theoxide coating is found to be a poor mask because most impurities readilydiffuse through a germanium oxide coating. Further, germanium oxide ispartially soluble in water, this being an undesirable feature of a mask.On the contrary, a coating of polycrystalline silicon dioxide formed ona semiconductor by oxidation at 1000 C. or above has excellentproperties of a diffusion mask. Only gallium penetrates apolycrystalline silicon dioxide coating.

To avoid problems of masking and finding suitable materials for masks,it is possible to use the oxide for the carrier of the impurities,hence, removing the need for a mask since the oxide acts both as a maskand a diffusant carrier. The use of this method not only protects thesemiconductor surface against contaminants, but helps prevent thedififusant from alloying with the surface, this sometimes occurring whena diffusant is deposited directly onto the surface and diffused therein.

It is therefore one object of this invention to provide a means fordiffusing an impurity into a semiconductor body.

Another object is to provide a means of forming a doped oxide on asemiconductor surface from a pyrolyti-c origin.

Still another object is to provide a means for selectively diffusing asemiconductor body.

Other objects and features of the invention Will become readily apparentwhen taken in conjunction with the appended claims and attached drawingin which:

FIGURE 1 is a schematic diagram of the apparatus used to form anddeposit the doped oxide onto the semiconductor body;

FIGURES 2a, 2b and 2c are cross-sectional views of a semi-conductor bodyshowing one possible embodiment made by the practice of the invention;and

FIGURES 3a, 3b and 30 show another semiconductor device in cross sectionillustrating another device made by the practice of the invention.

FIGURE 1 is illustrative of the type of apparatus which may be used inpracticing the method of the invention. Line 1 is from a source of argonand supplies argon gas through control valve 2, to vaporizing bottle 3containing 21 silane and a suitable dopant. From the bottle 3 theargon-silane-impurity vapor flows through line 4 to valve 5 whichcontrols and regulates the flow of vapor and valve 6 is a check valveallowing no vapor to flow back into bottle 3. Through line 7 flows ahigh purity commercial oxygen which is combined with the vapor mixturethrough valve 8. The vapor-oxygen mixture flows through valve 9 intoreaction chamber 10, which is closed gas tight except for the exhaustline through non-return valve 13. Within the chamber is a graphite boat11 upon which semiconductor wafer 12 is mounted.

In practicing the invention a germanium wafer or slice may be placedupon the boat 11 within the chamber 10. Argon and the impuritycontaining silane is passed through the chamber without the addition ofoxygen at ordinary room temperature to sweep the air out of theapparatus. The argon flow rate used is about 1-2 liters per minute. Thewafer and chamber are heated to 600 C. until the initial coating ofsilicon dioxide impurity is deposited on the wafer. The coating shouldhave a minimum thckness of at least 300-400 angstrom units, but becauseof the possibility of irregularities in a thickness of this silicondioxide coating, a thickness of 800 2000 angstrom units is preferred.After this first layer of silicon dioxide is formed, oxygen isintroduced in the chamber at a flow rate of about one cubic foot perhour along with the silane and the process is continued until a coatingof desired thickness of doped silicon dioxide is obtained.

Oxygen gas is not used initially in coating germanium because an oxideof germanium will form at a temperature of 450 C. and above. This oxideis not desired since it performs no useful function and iscontaminating. The silanes used contain sufficient oxygen so thatpolycrystalline silicon dioxide can be formed successfully at thetemperature employed.

The silanes employed in the process may be any of the oxy, organic oxycompounds of silicon such as ethylorthosilicate, ethyltrimethoxysilane,tetramethoxysilane, triethoxyethylsilane, triethoxymethylsilane, orethoxytriethylsil-ane, which are volatile under the conditions of theprocess. Any suitable dopant may be used in the silane such astrimethylborate. A mixture of this dopant and one of the silanes wouldresult in a boron doped silicon dioxide surface. After the doped oxidehas been deposited upon the surface of the semiconductor wafer, it isthen placed in the diffusion chamber and the temperature raised to about825 C., and held at this temperature until the dopant diffuses to thedesired dept Although argon has been employed as the carrier gas, otherinert gases such as helium, neon, zenon, and krypton may be used.Likewise, even though specific flow rates of the gaseous material used,almost any rate could be used which would allow decomposition of thesilane to occur. The reaction temperature is usually between about 600C. to 620 C., however, silicon dioxide will be deposited on a substrateas low as 575 C. to as high as about 900 C.

By way of an example, FIGURES 2a, 2b and 2c illustrate a device whichmay be made by practice of the invention.

FIGURE 2a shows a semiconductor 14 upon which a doped silicon oxidelayer '15 has been deposited upon the waferJIn FIGURE 2b a portion ofthe doped oxide 15 has been removed and another coating of undoped oxide16 has been grown thereon. After placing the wafer '14 in a diffusionfurnace, the diffused region 17 is formed. FIGURE 20 shows the waferafter the diffusion has taken place and an opening 22 is cut into theoxide 16 rernoving a portion thereof and all of the remaining oxide 15,exposing the diffused region in the device. A contact may be made to thediffused region and to one surface of-the wafer 14 resulting in a diodedevice.

Another device that may be made by the practice of the method isillustrated in FIGURES 3a, 3b and 3c. Shown in FIGURE 3a is asemiconductor wafer 18 upon which a doped silicon oxide layer 19 hasbeen deposited. FIGURE 3b illustrates the same wafer onto which an oxidecoating 20 has been deposited which is not impurity containing. Afterthis non-impurity containing oxide coating is placed on the wafer, thewafer may be placed into a diffusion furnace to form the diffusionregion 21. A portion of the wafer may be etched away leaving themesa-type structure shown in FIGURE 3c. The mesa-top portion 23 is thepart of region 21 'remaining after the etching step is performed and isformed by the dilfusionfrom the oxide impurity coating 19. The deviceshown in FIGURE 30 may be used as a diode or a subsequent diffusion maybe made, making another diffusion into the region 23 (not shown), thusforming a third region.

The temperatures and materials given have been disclosed for purposes ofillustration and should not be construed as placing undue limitationsupon the .invention, as many variations will be obvious withoutdeparting from the scope of the appended claims. Specific examples havebeen given showing structures which may be made by the practice of theinvention, but should not be construed in any limiting sense to be theonly structures that may be formed by the practice of the invention.

What is claimed is:

1. A method of selectively diffusing an impurity into a semiconductordevice comprising the steps of:

depositing an impurity containing oxide upon the surface ofsaid'semiconductor device, selectively removing a portion of said oxideto leave the desired configuration and diffusing the impurity into thedevice from the remaining oxide.

2. The method of doping a semiconductor device comprising the steps of:

exposing a semiconductor to an atmosphere of impurity containing siliconand argon in a reaction chamber, forming a coating of silicon dioxide onthe surface of said semiconductor device, said silicon dioxidecontaining said impurity and diffusing said impurity out of said oxideinto said semiconductor device.

3. The method according to to claim 2 wherein the temperature withinsaid reaction chamber is about 575 C. to 900 C.

4. The method according to claim 2 wherein the temperature of saiddilfusion is about 700 C. to 900 C.

'5. A method of making a semiconductor device comprising the steps of:

forming upon one face of a semiconductor substrate a first insulatinglayer containing a doping impurity, selectively removing a portion ofsaid first insulating layer, with the remainder of said first insulatinglayer overlying a region in said substrate, forming an undoped secondinsulating layer on said remaining portion of said first insulatinglayer and the face of said substrate exposed by the removal of saidportion of said first insulating layer, and heating said substrate to atemperature sufficient to diffuse said doping impurity from said firstinsulating layer into said region in said substrate to form a PNjunction.

6. A method of making a semiconductor device comprising the steps of:

passing argon through a vaporizing container containing silane and adoping impurity to form a first mixture, combining said first mixturewith oxygen to form a second mixture, flowing said second mixture overthe heated surface of a semiconductor substrate within a reactionchamber to form on one face of a semiconductor substrate a firstinsulating layer containing said doping impurity, selectively removing aportion of said first insulating layer with the remainder of said firstinsulating layer overlying a region in said substrate, forming anundoped second insulating layer on said remainder of said firstinsulating layer and said face of said substrate exposed by the removalof said portion of said first insulating layer, and heating saidsubstrate thus formed to a temperature sufiicient to diffuse said dopingimpurity from said first insulating layer into said region in saidsubstrate, thereby forming a PN junction.

7. A method of making a semiconductor device comprising the steps of:

passing argon through avaporizing bottle containing a silane and adopant to form a first mixture, combining said first mixture with oxygento form. a second mixture, flowing said second mixture over the surfaceof a semiconductor substrate within a reaction chamber thereby formingupon one face of a semiconductor substrate a first silicon oxide layercontaining a doping impurity, selectively removing a portion of saidfirst silicon oxide layer with the remainder of said first silicon oxidelayer overlying a region in said substrate, forming an undoped secondsilicon oxide layer on said remainder of said first silicon oxide layerand said face of said substrate exposed by the removal of said portionof said first silicon oxide layer, heating said substrate to atemperature sufficient to diffuse said doping impurity from said siliconoxide layer into said region in said substrate thereby forming a PNjunction, removing said remainder of said first silicon oxide layer andthe portion of said second silicon oxide layer on said first siliconoxide layer, thereby exposing a portion of the surface of said diffusedregion, and forming metallic contacts to said exposed portion of saiddiffused region and to the opposite face of said substrate.

8. A method of making a semiconductor device as in claim 7 wherein saiddoping impurity is boron and said substrate is heated to a temperatureof about 825 C. to diffuse said boron impurity from said silicon oxidelayer into said region in said substrate.

9. A method of doping a semiconductor device comprising the steps ofpyrolyticly depositing an impuritycontaining coating of silicon dioxideupon the surface of a semiconductor wafer, and diffusing said impurityfrom said oxide into said wafer.

References Cited UNITED STATES PATENTS 2,974,073 3/1961 Armstrong148-l89 2,804,405 8/1957 Derick 148-189 2,873,222 2/1959 Derick 148-187X 3,055,776 9/1962 Stevenson.

3,064,167 11/1962 Hoerni 148187 X 3,084,079 4/1963 Harrington.

3,085,033 4/1963 Handleman 148-191 3,200,019 8/ 1965 Scott.

3,203,840 8/1965 Harris 148187 7 5 HYLAND BIZOT, Primary Examiner.

1. A METHOD OF SELECTIVELY DIFFUSING AN IMPURITY INTO A SEMICONDUCTORDIVICE COMPRISING THE STEPS OF: DEPOSITING AN IMPURITY CONTAINING OXIDEUPON THE SURFACE OF SAID SEMICONDUCTOR DEVICE, SELECTIVELY REMOVING APORTION OF SAID OXIDE TO LEAVE THE DESIRED CONFIGURATION AND DIFFUSINGTHE IMPURITY INTO THE DEVICE FROM THE REMAINING OXIDE.